The present invention relates broadly to a method of and apparatus for treating an article in a plasma and, more specifically, to an improved method of and apparatus for dehydrating and/or consolidating glass soot for use in fabricating optical fibers.
It is recognized generally that optical fibers are a superior medium of communication. For example, optical fibers are smaller and lighter than copper wires. More importantly, a single fiber can carry hundreds of times more information than a simple metal wire can. Hence, compactness and high rates of transmission are commercially important features of a fiber optic system. However, fiber optic systems have yet to approach the cost-performance characteristics of copper systems. For fiber optic technology to become successful commercially it must be produced at a competitive price. Therefore, fabricating low cost optical fibers is key to providing a mass market for them.
Aside from cost considerations, another significant problem is quality. A major problem in the fabrication of optical fibers, especially of the type intended for long distance transmission, is to minimize optical losses to commercially acceptable levels. The purity required for achieving such low loss is achieved by sophisticated and time consuming techniques.
One of the two major techniques for fabrication of optical fibers is the so-called "soot deposition" process. Typically, in this process, glass precursor vapors are introduced into a hydrolyzing flame. The result is formation of adherent particulate material (i.e. soot) which is directed towards a mandrel upon which the soot adheres to form a soot preform. Following deposition, the soot preform is dehydrated and then consolidated into transparent fused silica. From a commercial standpoint this approach is desirable since the deposition rates are generally rapid. For example, a soot preform adequate for fabrication of a 20 kilometers of fiber may be prepared in a few hours. However, by virtue of the nature of the hydrolysis process there is formed impurities. Since formation takes place within a combustion environment contaminants are inevitable. One of the more troublesome is the hydroxyl radicals. These lead to light absorption peaks or zones in the fiber. Absorption, of course, leads to light loses during transmission.
For eliminating the hydroxyl material from the glass soot preform, the latter is subjected to a dehydrating process. Typically, dehydration occurs in a furnace heated to about 1000.degree. C. and which contains a gaseous drying agent, such as chlorine. The glass soot preform is introduced into the furnace whereat it is heated uniformly and the chlorine passes through the preform and effects removal of the water related impurities. The vessel walls forming the furnace are made of, for example, quartz to prevent contamination and the gas pressure is such as to resist tube collapse. In practice, the same furnace is used to consolidate the soot preform into transparent glass. Consolidation requires higher temperatures, for instance, in the range of about 1400.degree.-1700.degree. C. These consolidating temperatures are at about temperatures that quartz furnace walls soften. Moreover, these walls are subject to the corrosive effects of the gases which are sometimes used. In practice these walls are replaced often. Replacement of worn or deformed quartz furnace walls is time consuming and expensive not only because quartz is expensive, but there is significant downtime associated with each replacement. These costs add significantly to the overall production costs of the fibers. Known attempts to minimize the detrimental effect of heat on the walls include conducting heat away from the walls. Representative examples of the soot deposition technique are disclosed in the following U.S. Pat. Nos.: 3,806,570; 4,440,558 and 4,402,720.
The other approach is the so-called inside vapor deposition process, wherein glass precursor vapors are passed through a glass tube heated to very high temperatures as by a plasma. Glass particulate or soot is produced within the heated tube and accumulates on the tube inside whereat it is consolidated. The vapors are not subjected to a hydrolyzing flame so that the impurities normally associated therewith are substantially eliminated. Nonetheless, it is significantly more time consuming than the soot deposition technique. From a commercial standpoint, therefore, it is less desirable. Representative examples of such techniques are disclosed in the following U.S. Pat. Nos.: 4,217,027; Re. 30,635; 4,405,655; 4,262,035; 4,331,462 and 4,292,063.